Oriented graphitic structure



1955 L. R. M CREIGHT ORIENTED GRAPHITIC STRUCTURE Filed Oct. 18, 1962LOUIS R. M CRElGHT graphite in structures, such as rocket nozzles.

United States Patent Office 3,288,629 Patented Nov. 29, 1966 3,288,629ORIENTED GRAPHITIC STRUCTURE Louis R. McCreight, Wayne, Pa., assiguor toGeneral Electric Company, a corporation of New York Filed Oct. 18, 1962,Ser. No. 231,451 3 Claims. (Cl. 11746) This invention pertains to theproduction of articles of carbon, more particularly to the production ofarticles of the graphitic form of carbon known from its usual techniqueof formation as pyrolytic graphite.

In many applications Where moderate mechanical strength and the abilityto withstand high temperatures are requisite, graphite has beenemployed. Thus, graphite was a common material for members functioningin the blast from early rockets, of a decade ago. It has frequently beenincluded in designs for nozzles for rocket motors. (For example, US.Patent 2,958,- 184). However, microcrystalline graphite is subject tothe disadvantage of materials having random orientation of theircomponent crystals in that its strength is somewhat below thatultimately attainable, its physical prop erties such as thermalexpansion and conductivity are incapable of being favorably oriented fora particular use, and it is subject to erosion by flow ofhigh-temperature gases. Pyrolytic graphite, however, tends to bemacroscopically anisotropic. When deposited by pyrolysis of acarboniferous gas (for which a procedure will be described in detailhereinafter) it produces crystals whose a and b axes are parallel to thesubstrate, the c axis being normal to the substrate. Since the crys-'talline structure and orientation of the deposit are not continuationsof a structure of the substrate, but quite independent thereof, thisphenomenon is not epitaxy. Because it is a self-orientation of thematerial it may reasonably be called idiotaxy, and the material bedescribed as idiotaxial. Since the phenomenon obviously does notdetermine the orientation of the axes parallel to the surface of thesubstrate but only of the axis normal to the substrate, the orientationis defined only as regards the normal axis, the c axis in the case ofpyrolytic graphite. Several suggestions have been made for theutilization of the anisotropic properties of pyrolytic Nozzle throatshave been coated, by pyrolysis, with pyrolytic graphite oriented withits a and b axes parallel to the coated surface, and their axes normalto the surface. It has been found that this tends to produce failure bybuckling or blistering at the nozzle throat where the effects ofvelocity and temperature, combined, are a maximum. Alternatively, it hasbeen proposed to form a nozzle of disks of pyrolytic graphite held in astack with their a and b axes parallel to the plane of the disk surfaceand the c axis pointing in the direction of the adjacent disks. This hasgood mechanical results; but the thermal conductivity along the a and baxes is much greater than that along the c axis. In consequence, heatleakage through the graphite rings to the substrate is excessive, to thedamage of the substrate. In view of the cited tendency for pyrolyticgraphite to be formed with a and b axes parallel to the surface uponwhich deposition occurs, it would appear to be difficult in the light ofthe known art to find any alternative to the two schemes described.

I have invented a way of lining a nozzle throat with arbitrarilyoriented pyrolytic graphite, desposited upon the substrate, without thenecessity of intermediately forming large disks for subsequent assemblyand machining. While this method is particularly well adapted to producea novel improved rocket nozzle, it is equally applicable to produceother products of this new kind.

I achieve this by extending from the substrate a plurality ofprotrusions, which may be rod-like or layered projections, in the areawhere I desire the a and b axes of the deposited graphite to be otherthan parallel to the main surface of the substrate. Deposition occurswith a and b axes of the graphite parallel to the surface of the rods,or layers, which surfaces are themselves nonparallel to the main surfaceof the substrate.

For the better understanding of my invention I have provided figures ofdrawing, in which FIG. 1 represents in section a rocket nozzle with alining of pyrolytic graphite, deposited in situ;

FIG. 2 represents in section a rocket nozzle formed of stacked graphitedisks;

FIG. 3 represents in section a rocket nozzle embodying my invention; and

FIG. 4 represents apparatus for carrying out a deposition of pyrolyticgraphite to form a variously oriented coating, according to myinvention.

Referring first to FIG. 1, there is represented a body portion 10 whichis of some refractory material, either ceramic, graphite, metal, or acombination of these. Its inner surface is represented covered withparticles of pyrolytic graphite 12, which are represented by straightlines, similar to the official symbol for the section of a ceramic, butlying parallel to the face of body 10. When pyrolytic graphite is formedupon a surface by methods to be described in detail hereinafter, itforms platelets of crystal whose a and b axes lie parallel to thesurface of deposition, or substrate. The direction of the lines,therefore, is indicative of the plane of the a and b axes of thepyrolytic graphite 12. This deposit 12 forms a nozzle passage having anentrance 14, and exit 16, and a thorat section 18. This form of rocketnozzle, known previously to my invention, has the advantage thatpyrolytic graphite is refractory and is not readily eroded by the flowof hot gases. However, it has been found by experience that there is atendency for a layer of pyrolytic graphite 12 as here represented toform blisters such as 20, tearing loose from its substrate, withconsequent loss of mechanical strength, deformation, and consequentshortening of its useful life.

There is represented in FIG. 2 a structure which has been employed,previously to my present invention, to overcome some of the difficultiesdescribed as occurring in the embodiment of FIG. 1. In FIG. 2, there isrepresented a sleeve-like outer housing 22, which may conveniently be ofmetal. This sleeve holds a stack of disks 24 through 34 inclusive, whichare formed by deposition upon a flat substrate and in consequence havetheir a and b axes located parallel to the upper or lower surfaces ofthe disk. The disks are mounted so that, when stacked, they form achamber or passageway having entrance 14, exit 16, and throat 18 of thesame shape as represented in FIG. 1. This type of structure has beenfound to possess some satisfactory properties, in that the ends of the aand b axes of the graphite are the ones primarily exposed to the actionof the hot gases. Such exposure has been found to afford the highestpossible resistance to erosion and abrasion by the hot gases. However,the heat transfer in the a and b directions is also a maximum. Inconsequence, it has been found that the heat losses from such a nozzlenot only tend to reduce efficiency but are so great that they tend to beinjurious to the outer housing 22.

It would be desirable to enjoy the benefits of poorer heat conductivityin the coating in entrance 14 and exit 16 and tolerate high heatconductivity in return for abrasion resistance only in the region of thethroat 18 where such resistance is most urgently required, because ofthe higher gas velocity. FIG. 3 represents how I have 3 achieved this.Outer housing 36 is of some convenient refractory material which may beany one of those listed as possible for item of FIG. 1. In the vicinityof the throat 18, there are inserted in housing 36 pins 38, which may beof some suitably refractory metal such as molybdenum. When thedeposition of pyrolytic graphite upon the interior of housing 36 isconducted, the coatings 40 and 42 lie with the a and b axes parallel tothe substrate. However, deposition in the vicinity of the throat 18occurs primarily upon pins 38, producing a deposit 44 whose a and b axeslie parallel to the pins 38 and therefore normal to the surface ofhousing 36. Thus, as may be seen by reference to FIG. 3, I produce anozzle having a lining of pyrolytic graphite, having low heatconductivity in the vicinity of the entrance 14 and the exit 16, buthaving higher abrasion resistance in the vicinity of the throat 18. Itis, of course, apparent that the pins 38 need not be cylindrical, butmay equally well be flat strips having their flat surfaces facing thetop and bottom of the nozzle. Obviously, some crystal growth will occurfrom the substrate itself until it is stopped by interference withcrystals growing out from the protruding pins. (This phenomenon isclassically illustrated in every treatise on elementary metallurgy as anexample of how crystals grow from the sides and bottom of an ingotduring freezing) If the crystals growing out from the protruding pinsactually sealed off the substrate hermetically from access of thecarboniferous gas before the substrate was solidly coated with graphite,there would, obviously be voids in the graphite immediately adjacent tothe substrate. Suc- 'cinctly, for a given spacing between pins: 'If thepins are too long, the graphite would not contact the refractory base,and if the pins are too short, the orientation would be parallel to thebase. However, it is obvious by elementary kinematics (in addition tothe teachings of elementary metallurgy) that materials growing at thesame speed from two surfaces at an angle to each other will intersectalong a plane which bisects the angle between them. Thus for uniformgrowth rates, the height of undesired growth from the substrate cannotbe more than half the spacing between adjacent pins; clearly any pinlength greater than this will result in an upper surface which has grownexclusively from the pins. Actually, the sensitivity of deposition rateto temperature makes these considerations somewhat academic. If heat isapplied via the substrate, the rate of growth at the substrate may firstexceed that at the pins if these rise more slowly in temperature, sothat the deposition will first Occur on .the substrate and the parts ofthe pins near the substrate.

. ing are used.

FIG. 4 represents schematically the basic elements of apparatus suitedfor the deposition of pyrolytic graphite. There is represented a chamber46 having the cover 48 which may be hermetically sealed to it, forming aclosed chamber. From a source of hydrocarbon gases at high pressure, notrepresented, a tube or pipe leads to reducing valve 50 which isconnected by pipe or tube 52 to discharge gas into the interior ofcontainer 46. A pressure gauge 54 is connected by a pipe 56 to measurethe pressure in the interior of container 46. In order to provide forpossible operation at less than atmospheric pressure, there is alsoconnected to container 46 a valve 58 which is connected to an exhaustpump 60,:which discharges to the atmosphere. Inside of container 46 andresting upon its bottom, there is a refractory support 62, which may bea piece of ceramic of any convenient kind, since it serves only as asupport. A rectangle 64 represents a housed induction coil of aninduction furnace inside or around which a conductive piece such, forexample, as that represented in FIG. 1 or FIG. 3 may be placed,providing that the material 10 or 36, respectively, is conductive, suchas a metal or graphite. Terminals from the coil in housing 64 areconnected by conductors 66 to feed-through terminals 68, which carry theelectrical circuit through the wall of container 46, and are furtherconnected by conductors 70 to a tapped coil 72 which is connected toterminals 70, which are connected to a source of high frequency energynot shown. By adjusting the tap on coil 72, the energy input to the bodyplaced inside 64 may be adjusted. Reference 76 represents thermocoupleleads which pass through stuffing box 78 through the wall, of container46 to millivoltmeter 80, whose reading will give an indication of thetemperature of the junction of the thermocouple. The thermocouplejunction may be placed in contact with the work piece to be heated inorder that its temperature may be measured. If the work piece is made ofa non-conducting material, a suitable conductive coating may be appliedto its interior portions, as by spraying a thin layer of molybdenum uponit by known metal-spraying processes; -or,-alternatively, a simplefurnace element may be placed inside shield 64 and the ceramic piece maybe heated directly as in a mufile furnace.

Direct resistive heating is a third alternative. A published referenceupon the deposition of pyrolytic graphite is to be found in IndustrialCarbon and Graphite, published in 1958 by the Society of ChemicalIndustry, London, S. W. 1. On pages 86 through of this volume, there maybe found a paper by Brown and Watt entitled The Preparation andProperties of High-Temperature Pyrolytic Graphite. In accordance withthe teaching of this paper, pyrolytic carbon may be deposited at a rateof about 25x10 centimeters per second of deposit thickness growth upon asubstrate maintained at a temperature of 2100 C., by providing anatmosphere of methane at a pressure of 15 centimeters of mercury (page88, Table I). This procedure results in the production of a graphitedeposit of density approximately 2.2 grams per cubic centimeter. Thismay be performed with the apparatus of FIG. 4 by placing the work pieceinside the heating device represented by 64, opening valve 58 andoperating pump 60 to substantially exhaust the container 46, topped byclosure 48; and adjusting valve 50 to maintain a reading equivalent to15 centimeters of mercury upon pressure gauge 54. Energy may be appliedto terminal 70 and the tap on inductor 72 may be adjusted as required toproduce a temperature of approximately 2100 C. as indicated by thereading of millivoltmeter 80. An alternative, sometimes preferable,method of determining the temperature of the work piece is to providecover or closure 48 with a suitable window to permit observation of thework piece and the measurement of its temperature by means of an opticalpyrometer. The techniques of heating materials to high temperatures ingaseous atmospheres at moderate pressures and measuring theirtemperatures are all very Well known, and there are numerous waysavailable from the art for performing each of the functions described.The particular procedure described here has been included pro forma forcompleteness of disclosure. In actual practice, convenience andavailability of equipment are more likely to determine .the choice ofthe particular procedure used than any particular technicalrequirements. It may be observed from .the reference that pyrolyticgraphite may be deposited at a number of different temperatures and at anumber of different pressures from a number ofdilferentcarboniferousgases. The Brown and Watt reference teaches this, and specificallyindicates the idiotaxial nature of the deposits at page 99 thereof,under Conclusions (2) with the statement, The deposited carbons have astructure of hexagonal layer planes of carbon atoms as in graphite, thelayer planes all lying parallel to each other and the surface of thesubstrate but being otherwise randomly oriented. My invention professesonly to make use of the techniques for depositing pyrolytic graphite andto teach a method of producing a desired result in the course of suchdeposition; it is pointed out that the deposition of pyrolytic graphiteas such is part of known art, including additions such, e.g., as boron.

Returning once more to a consideration of FIG. 3, it is evident in thelight of the description I have just given of the procedure fordepositing pyrolytic graphite that as the carbon bearing gas comes incontact with the internal surface of 36 and the surfaces of pins orstrips 38, the deposition of pyrolytic graphite will occur upon theexposed surfaces and the thickness of the layer will increase by furtherdeposition upon the graphite already deposited. Thus, the graphitedeposited at points 42 and 40, that is, the entrance and exit of thenozzle, will simply grow thicker, but the deposition occurring in thethroat region 18, represented by 44, will begin primarily upon thesurfaces of pins or strips 38 and will extend until it forms a bridgefrom one pin to the next and will continue until the interstices arefilled, so that the graphite represented by reference number 44 will becompact and dense comparably with the density of the graphiterepresented by reference numbers 40 and 42, differing therefromprimarily in its crystal orientation. The junctions between the graphitemasses 40, 42 and 44 will, of course, also be filled by deposition ofgraphite through decomposition of the carbon bearing gas. Therefore,there will be found a continuous mass of graphite, having theorientation of its crystals arbitrarily controlled, being different indifferent places. Since it may not be possible to achieve suflicientuniformity of the deposition process to produce a completely symmetricalnozzle by the procedure I have described, I envisage the possiblenecessity of finishing the surface of the graphite so deposited byordinary machining processes. While my invention has been described withrespect to embodiment in a rocket nozzle, it is obvious that it may beusefully applied in any application where graphite as such may be used.Such applications, based upon my teaching, lie within ordinary skill inthe art.

The appended claims are written in subparagraph form, in compliance witha recommendation of the Commissioner of Patents, to render them easierto read. This particular manner of division into subparagraphs is notnecessarily indicative of a particular relative importance or necessarysubdivision of the physical embodiment of the invention.

What is claimed is:

1. The method of forming deposits of pyrolytic carbon so attached to asubstrate that the normal axes of the deposit are at a predeterminedacute angle with the surface of the substrate which comprises:

providing the said substrate with protrusions each of circular symmetryaround a central axis,

the central axis of each protrusion making with a normal to the surfaceof the substrate an angle equal to the said predetermined angle,

said protrusions having lengths sufiicient to ensure orientationparallel to the surface of the protrusions in the interstices betweenthe protrusions,

said protrusions also having limited space therebetween to prevent anysubstantial amount of orientation parallel to the surface of thesubstrate,

causing the deposition of pyrolytic carbon by destructive decompositionof carboniferous gases to occur on the said protrusions until theinterstices between the protrusions are filled with pyrolytic carbon.

2. The method claimed in claim 1 in which the therein said central axesof the protrusions are all parallel to each other.

3. The method of forming deposits of pyrolytic carbon so attached to asubstrate that the normal axes of the deposit are at a predeterminedacute angle with the surface of the substrate which comprises:

providing the said substrate with protrusions having surfaces of singlecurvature whose generating straight lines intersect the surface of thesubstrate at an angle which is the complement of said predeterminedangle,

said protrusions having lengths sufiicient to ensure orientationparallel to the surface of the protrusions in the interstices betweenthe protrusions,

said protrusions also having limited space therebetween to prevent anysubstantial amount of orientation parallel to the surface of thesubstrate,

causing the deposition of pyrolytic carbon by destructive decompositionof carboniferous gases to occur on the said protrusions until theinterstices between the protrusions are filled with pyrolytic carbon.

References Cited by the Examiner UNITED STATES PATENTS 2,217,193 10/1940 Aronson. 2,334,257 10/ 1943 Egger et al. 2,614,947 10/ 1952Heyroth. 2,789,03 8 4/1957 Bennett et a1. 2,849,860 9/1958 Lowe.2,957,756 10/ 1960 Bacon 23209.2 2,958,184 11/1960 Sanders. 2,987,874 6/1961 Nicholson. 3,070,957 1/ 1963 McCorkle. 3,156,091 11/1964 Kraus.

FOREIGN PATENTS 599,275 3/ 1948 Great Britain.

OTHER REFERENCES Aviation Week, Dec. 7, 1959, pp. 99 and 101 relied on.Aviation Week, Feb. 13, 1961, pp. 67, 69, 71, 72 relied Pyrographite, byRaytheon Co., received Aug. 17, 1961, pp. 113 relied on.

ALFRED L. LEAVITT, Primary Examiner.

ALAN BLUM, RICHARD D. NEVIUS, MURRAY KATZ, Examiners.

C. R. CROYLE, A. H. ROSENSTEIN,

Assistant Examiners.

1. THE METHOD OF FORMING DEPOSITS OF PYROLYTIC CARBON SO ATTACHED TO ASUBSTRATE THAT THE NORMAL AXES OF THE DEPOSIT ARE AT A PREDETERMINEDACUTE ANGLE WITH THE SURFACE OF THE SUBSTRATE WHICH COMPRISES: PROVIDINGTHE SAID SUBSTRATE WITH PROTRUSIONS EACH OF CIRCULAR SYMMETRY AROUND ACENTRAL AXIS, THE CENTRAL AXIS OF EACH PROTRUSION MAKING WITH A NORMALTO THE SURFACE OF THE SUBSTRATE AN ANGLE EQUAL TO THE SAID PREDETERMINEDANGLE, SAID PROTRUSIONS HAVING LENGHTS SUFFICIENT TO ENSURE ORIENTATIONPARALLEL TO THE SURFACE OF THE PROTRUSIONS IN THE INTERSTICES BETWEENTHE PROTRUSIONS, SAID PROTRUSIONS ALSO HAVING LIMITED SPACE THEREBETWEENTO PREVENT ANY SUBSTANTIAL AMOUNT OF ORIENTATION PARALLEL TO THE SURFACEOF THE SUBSTRATE, CAUSING THE DEPOSITION OF PYROLYTIC CARBON BYDESTRUCTIVE DECOMPOSITION OF CARBONIFEROUS GASES TO OCCUR ON THE SAIDPROTRUSIONS UNTIL THE INTERSTICES BETWEEN THE PROTRUSIONS ARE FILLEDWITH PYROLYTIC CARBON.